Asexual embryogenesis in plants

Abstract: The capacity of somatic cells and pollen grains or microspores to undergo embryogenesis is an important property used in improvement and regeneration of plants. Furthermore, somatic embryogenesis as well as pollen embryogenesis show many similarities with zygotic embryogenesis and are, thus, good model systems. Pollen suspensions of tobacco (Nicotiana tabacum L.) contain a mixture of embryogenic and normal pollen grains of different developmental stages. They could be fractionated due to different surface properties by using aqueous two-phase partition. Cold led to a general increase in specific isoperoxidase activity in pollen fractions enriched in embryogenic pollen, and also to changes of the isoperoxidase pattern towards more neutral and alkaline types. The total change of peroxidase activity obtained after cold-treatment was most pronounced in embryogenic pollen, and was probably both due to a general stress reaction in all pollen and to a switch into the sporophytic pathway of embryogenic pollen. There were also clear differences in peroxidase activities between embryogenic and non-embryogenic somatic carrot cells (Daucus carota L). The major part of the peroxidases was soluble with about three times lower specific activity in embryogenic cells as compared to non-embryogenic cells. The activity of membrane-bound peroxidases was enriched in the intracellular membrane fractions but, as opposed to the soluble peroxidases the activity was higher in embryogenic cells. The dominating Ca2+ transport system in both embryogenic and non-embryogenic carrot cells was a primary ATP-dependent Ca2+ transport located in the plasma membrane. The Ca2+ transport of plasma membrane vesicles was higher in non-embryogenic cells, although the uptake in vesicles from intracellular membranes was higher in preparations from embryogenic cells. Using a Spearman rank correlation test a negative correlation was found between plasma membrane-bound Ca2+ transport and intracellular membrane-bound peroxidases, which is in agreement with the fact that increased cytosolic Ca2+ activates the secretion and the activity of peroxidases. The alterations in peroxidase activity and Ca2+ transport thus appeared already before embryo development took place (the carrot cell suspensions did not contain any embryos), and probably reflected processes in the apoplast linked to cell wall formation and important with regard to cell shape determination. In pollen, specific peroxidases were linked to embryogenic pollen and it is possible that also in somatic cells the induction of embryogenesis involved specific peroxidases. Subcellular studies on enzyme activities give valuable contributions to the understanding of developmental processes during embryogenesis.